Asmodeus rules over the Nine Hells/Baator in their entirety - all of the other Archdevils answer to him. Which of the other Outer Planes, if any, are ruled/overseen by a single god or powerful entity?
Answers should ideally be sourced from 5e materials, but given the lack of detailed information about the planes in 5e at the moment, it would also be helpful to know answers from previous editions.